The motor graders of some types include a torque converter provided with a lock-up clutch (see Japan Laid-open Patent Application Publication No. JP-A-2000-320362). When the lock-up clutch in such a motor grader is set to be in an engaged state, an operator can feel a direct speed control that the vehicle speed is changed in accordance with an acceleration control by the operator. When the lock-up clutch is set to be in an engaged state, however, the input side and the output side of a torque converter are set to be in a directly engaged state. Therefore, an engine stall may be caused when a large load is applied to the output side.
Meanwhile, Japan Laid-open Patent Application Publication No. JP-A-H05-272636 describes a technology of a large agricultural tractor for avoiding occurrence of an engine stall, although this is not related to the motor graders. In the technology, the lock-up clutch is changed into a disengaged state when the revolution of an input shaft in the lock-up clutch becomes less than or equal to an engine idle revolution (i.e., a low idle revolution) during an engaged state of the lock-up clutch. Accordingly, occurrence of an engine stall is avoided.